Presently, during the rescue processes of various accidents and natural disasters, in order to improve the rescue efficiency, a comprehensive coordinating command system shall be provided as well as adding manpower and materials. The current dispatching command system transmits information mainly using communication tools such as cell phones, which are usually influenced by many factors such as weather, geographical position and high cost and only unilaterally reflect the voice information. Thus the commander can only acquire the site conditions through hearing to make a decision and an analysis. As a result, the instruction issued by the commander is often lacks accuracy, timeliness and security. Therefore, it is emergently needed to develop a dispatching command system used in rescuing for achieving simple operation, a long conversation time, a high tone quality and a free call charge, and based on a simple visual long distance radio communication device.
The mobile communication technique develops always rapidly since it occurs, and has come through three generations till now. The first generation (1G) mobile communication system originates from 1980's, and it mainly uses the simulation technique and the Frequency Division Multiple Access (FDMA) technique. Due to the limitation of the transmission bandwidth, the fatal defect is that a long-distance roaming of the mobile communication cannot be carried out. The second generation (2G) mobile communication system originates from 1990's, and it mainly uses the digital Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA) techniques. This system realizes voice and data services by means of data transmission. The third generation (3G) mobile communication system develops more quickly, and substantially achieves the technical capability aims of “global unified standard, unified band and global seamless roaming”, “high spectral efficiency” and “mobile multimedia service”. The commercialized WCDMA achieves 384 kbps, CDMA 2000 1X achieves an average user rate of 50 kbps to 97 kbps, and 1XEV/DO achieves an average user rate of 500 kbps. Currently the rate realized by GPRS is 30 kbps to 40 kbps. The experimental system of TD-SCDMA reaches a rate of 148 kbps. It can provide various broadband information services that cannot be provided by the 1G and 2G products, such as high rate data, slow rate image, TV image, etc.
The radio signal four-wire duplex transmission technique completely separates the transmission and reception from each other, like making a phone call, and it is more convenient than the half-duplex interphone. The data transmission and reception are separately performed through two different transmission lines. The two communication parties can perform the transmission and reception at the same time, and the transfer mode is full duplex. Under the full duplex mode, each end of the communication system is provided with a transmitter and a receiver, thereby the transfer of data in two directions can be controlled simultaneously. The full duplex mode needs not to switch the direction, thus there is no time delay caused by the switching operation, which is very beneficial to interactive applications not allowing time delay (e.g., remote monitor and control system). This mode requires each communication party to have a transmitter and a receiver, and needs two channels for transferring data signals, i.e., a four-wire transmission.
The digital communication technique has a strong anti-interference capability. The information of the digital communication is contained in the existence of the pulse, and so long as the noise absolute value does not exceed a certain threshold value, the receiving end can determine whether a pulse is existed, to ensure the communication reliability. In addition, the digital communication technique can ensure the quality of a long distance transmission, because a regenerative relaying is adopted in the digital communication and noises can be eliminated. The regenerated digital signal is the same as the original digital signal and can be continued to be transmitted, thus the communication quality is not influenced by the distance, and a long distance communication of high quality can be carried out. Moreover, the digital communication technique also has advantages such as being suitable to various communication service requirements (e.g., telephone, telegram, image, data, etc.), as well as being convenient to realize a unified Integrated Service Digital Network (ISDN), to use a large-scale integrated circuit, to perform an encryption processing and to achieve the computer management of the communication network.
The ultrahigh sensitive signal receiving technique of −100 dBm can solve the bottleneck problem that a transmission distance of an ordinary Bluetooth communication is only a dozen of meters, decrease the acceptable threshold below −100 dBm, and realize the remote transmission of the sound narrow band. The so called threshold effect is a phenomenon that the output Signal-To-Noise Ratio (SNR) of an envelope detector is rapidly deteriorated after the input SNR of the detector is decreased to a specific value. The input SNR at which the threshold effect occurs is called as a threshold value. The threshold effect is caused by the nonlinear demodulation of the envelope detector. In case of a small SNR, the modulation signal cannot be separated from the noise, and the useful signal is submerged in the noise. At that time, the output SNR of the detector is rapidly deteriorated rather than being decreased with the input SNR in proportion, that is to say, a threshold effect occurs.
The work principle of the Frequency-Hopping Spread Spectrum (FHSS) technique is a communication mode where the carrier frequency for signal transmission between the receiving and transmitting parties is changed discretely in a predetermined rule, i.e., the carrier frequency used in the communication randomly hops under the control of the pseudo-randomly variable code. Among the implementation ways of the communication technique, “frequency hopping” is a communication mode that performs multi-frequency frequency-shift keying using a code sequence, and it is also a communication system with carrier frequency hopping under a code control. Viewed from the time domain, the frequency-hopping signal is a multi-frequency frequency-shift keying signal; and viewed from the frequency domain, the spectrum of the frequency-hopping signal randomly hops in an irregular interval within a very wide band. In which, a frequency-hopping controller is the core component, with the functions of frequency-hopping pattern generation, synchronization, adaptive control, etc.; a frequency synthesizer synthesizes the required frequency under the control of the frequency-hopping controller; and a data terminal performs a data error control. As compared with the frequency-fixed communication, the frequency-hopping communication is concealed and difficult to be captured, thus it has a strong anti-interference capability. Even if some frequency points are interfered, a normal communication still can be carried out at other frequency points not interfered. Since the frequency-hopping communication system is an instantaneous narrow band system, it is easy to be compatible with other narrow band communication systems, i.e., the frequency-hopping radio station can be exchanged with conventional narrow band radio stations, which is beneficial to the device update. Frequency-hopping is one of the most common ways for spectrum spreading, and it is also widely used in many fields such as GSM, wireless local area network, indoor wireless communication, satellite communication, underwater communication, radar and microwave.
The 2.4 GHz wireless technique is a short distance wireless transmission technique, with the advantages of high bandwidth (2 Mbps), bidirectional transmission, strong anti-interference capability, far transmission distance (within the range of the short distance wireless technique) and low power consumption. It is well known that 900M is the band of the public network, thus signals can be transmitted by means of the public base station. 1.8G band is the public network dual-frequency backup band, and 2.4G band is the ISM band commonly used in the world, thus they can be used free of charge without applying for a license.
In the data communication system or computer network system, the bandwidth or capacity of the transmission medium usually exceeds the requirement of a single signal transmission. In order to effectively utilize the communication lines, it is hoped that multiple signals are simultaneously transmitted in one channel, which is the so called multiplexing technique. With the multiplexing technique, the signals can be combined and transmitted in one physical channel, so as to greatly save the cable mounting and maintaining fees during the long distance transmission.